Quote:
Originally Posted by phosfiend
I've got a lovely old SONY TA-45 integrated amp (late 70's early 80's I believe) and it has this magical "loudness" button. Is this a compressor circuit? Is a preset EQ adjustment? I also have a modern AV amp with a "midnight" setting which I have a feeling is similar, but digital, and applied on a dynamic scale based on volume - more "midnight" at lower volumes and less at high.
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As others have explained, the "loudness" button is simply a bass boost (and sometimes a treble boos) designed to compensate for the ear's inability to perceive bass at low volumes (the Fletcher-Munson curves). A really well designed loudness circuit should gradually reduce the EQ as the volume control is increased, but not all are well-designed.
The "midnight" or "dynamic range" or "compression" settings on Dolby Digital receivers is something completely different. Dolby Digital allows recording unprecedented dynamic range. The loudest sounds at 105 dB and the quietest sounds at 25 db in a properly calibrated Dolby Digital playback system. That is just incredibly demanding of the system. It is also very loud on the explosion scenes!
There are two problems. If you turn down the system so the explosions aren't too loud, you can no longer hear the dialog properly, let alone the very quiet sounds. The second problem is that most systems can't handle that kind of dynamic range.
So Dolby has included a really sophisticated (and amazingly capable) dynamic range compression scheme that allows the same recording to be played back at full dynamic range on stout systems, at moderate dynamic range (similar to VHS Hi-Fi) for average systems, and at low dynamic range for modest systems (like a boxed system or TV speakers).
The way it works is that instructions for how to compress the recording are encoded as metadata when the recording as made (much as song information and album art is stored as metadata when you encode an MP3). Then, depending on the settings of your Dolby Digital decoder, you get full dynamic range or use the pre-encoded instructions to get minimum dynamic range or something in between.
The actual compression is quite sophisticated. Average level sounds (like movie dialog) aren't changed at all. The loudest sounds are reduced to a fixed limit. The quietest sounds are increased to a fixed minimum. Sounds in between are adjusted at variable rates. Very, very slick to have a single recording that can be played with unprecedented dynamic range on monster system or scaled back to also work just fine on a rinky-dink TV speaker system.
The "midnight" button on your receiver is one of these compression settings -- usually the one that mimics the dynamic range of a normal TV broadcast (usually about 40dB difference between the loudest and softest sounds compared to the 80 db difference with no compression. Many receivers give you a range of choices in between --slight compression like a CD, slightly more like a VHS Hi-fi, or even more like an FM or analog TV broadcast.
I should add one thing: since the "midnight" or "compression" modes are part of the Dolby Digital decoder software, they usually only work on Dolby Digital recordings -- not DTS, not FM, not CDs, and not analog TV.
